1. Field of the Invention
This invention relates generally to the field of electrical energy conversion systems and more particularly to electrical inverter circuits utilizing a solid state active element oscillator of the multivibrator type to convert DC voltage into AC voltage.
2. Description of the Prior Art
Inverter circuits, as considered herein, are designed to convert a DC input voltage into a high frequency alternating output voltage. The push-pull types of inverter circuits are recognized as the most efficient as a class for this purpose. However, even these circuits are plagued with a number of identifiable inefficiencies. The most significant of these is the energy loss that occurs due to common-mode conduction. This occurs when both transistors conduct simultaneously. The simultaneous conduction in turn is the result of an inherent and unavoidable delay associated with the turn-off action of applicable transistors. Such transistors normally do not have a corresponding delay associated with the turn-on action. Attempts have been made to correct this problem by selecting transistors with the lowest possible turn-off delay but this approach has required the use of very costly transistors.
A second major cause of energy loss in such a circuit is the power dissipation that occurs within each transistor during its turn-off transition. To minimize this loss, it is important to operate each transistor at near its maximum switching speed. However, it is even more important to prevent the collector voltae from rising significantly before the transistor has been fully turned off.
A third significant cause of energy dissipation results from turning on a transistor before its collector voltage has been reduced to its minimum level. This reduction of collector voltage occurs after the other transistor has been turned off, and as a result of its rising collector voltage.
Another cause of energy loss results from power dissipation within each transistor while it is conducting. To minimize this loss, it is necessary to provide adequate base drive corresponding to the collector current flowing at any given time. However, if this base drive is in excess of what is required to control the transistor, it can in itself become a cause of unnecessary power loss.
A great deal of design work on push-pull inverter circuits has been performed by others and reported in the patent art. Examples of such circuits are described in the patent to Jensen U.S. Pat. No. 2,997,664 entitled "Saturable Core Transistor Oscillator"; the patent to Wellford U.S. Pat. No. 3,248,640 entitled "Synchronizing Circuit"; the patent to Mehwald U.S. Pat. No. 3,324,411 entitled "Transistor Inverter with Inverse Feed-Back Frequency Stabilization Control"; the patent to Bishop et al, U.S. Pat. No. 3,461,405 entitled "Driven Inverter Dead-Time Circuit"; the patent to Paget, U.S. Pat. No. 3,579,026 entitled "Lamp Ballast"; the patent to Low, No. 3,663,944 entitled "Inverter Oscillator with Voltage Feed-Back"; the patent to Cox, U.S. Pat. No. 3,691,450 entitled "Power Inverter Oscillator Circuit"; the patent to Hook, U.S. Pat. No. 3,913,036 entitled "High Power High Frequency Saturable Core Multivibrator Power Supply"; and the patent to Ghiringhelli, U.S. Pat. No. 4,016,477 entitled "Novel Multipath Leakage Transformer and Inverter Ballast". The above identified patents have recognized some of the causes of power loss within a multivibrator inverter circuit and have proposed solutions for partially correcting for these losses. However, none have recognized all of the causes of such power loss as identified herein nor have they suggested solutions for eliminating such power losses within an economical and operationally effective frame of reference. Some also employ independent oscillator or drive circuits which add to the total cost of the inverter and may be wasteful of energy in and of themselves.
Another approach to the solution of most of the identified problems has been described in my pending application entitled "High Efficiency Push-Pull Inverters", filed Mar. 20, 1978, and assigned Ser. No. 890,586. This invention employs a saturable inductor across the base-emitter junction of each transistor for providing, when saturated, a near-short circuit path for the rapid evacuation of the charge carriers stored in the junction, and thereby ensures the rapid turn-off of the respective transistor.